PCNA Ubiquitination Is Important, But Not Essential for Translesion DNA Synthesis in Mammalian Cells
Figure 1
UV sensitivity of mouse embryo fibroblasts carrying the PcnaK164R/K164R mutation.
(A) Outline of ubiquitination and deubiquitination reactions of PCNA. Treatment with DNA-damaging agents, such as UV light, induces monoubiquitination of PCNA at Lys164 by the Rad18 E3 ligase. A common model suggests that the monoubiquitinated PCNA (PCNA-mUb) directly recruits TLS polymerases enabling translesion DNA synthesis. The deubiquitinating enzyme Usp1 removes the ubiquitin from PCNA-mUb, thereby negatively regulating the level of PCNA-Ub. The Rad5 homologs Shprh and Hltf, and an unidentified additional E3 ligase (marked x?) can extend the ubiquitin chain of PCNA-mUb, and the polyubiquitinated PCNA (PCNA-polyUb) thus formed promotes homology-dependent (template switch) error-free damage tolerance. (B) Pcna+/+ and PcnaK164R/K164R MEFs were irradiated at the indicated UV doses, and assayed for viability after 48 hours by measuring the level of cellular ATP. Each point represents the average of 3 independent experiments.